Thinning (also referred to herein as “grinding”) of semiconductor wafers has conventionally been accomplished by “back side grinding” methods whereby the circuit side of a wafer is protected with a surface protecting sheet and the back side opposite the circuit side is ground. The thicknesses of silicon wafers are generally 150 μm on an industrial level, but even thinner wafer thicknesses are desired. When wafers are ground to even smaller thicknesses, the phenomenon of unevenness of the ground surface (back side) due to irregularities on the circuit side, i.e., the phenomenon of transfer of the circuit pattern to the back side, becomes more notable. The cause of this back side transfer phenomenon is explained as follows. Presently employed pressure sensitive adhesive surface protecting sheets are limited in their ability to conform to the irregularities of the circuitry on a semiconductor wafer. As a result, gaps (air pockets) are created between the pressure-sensitive adhesive layer and the circuit side, such that the wafer is not directly supported by the pressure-sensitive adhesive (protective layer) in those areas. When the wafer is ground very thin, the wafer shifts in the vertical direction at the unsupported scribe lines (street) between the circuit die while compressing the air pockets with the result that the wafer is not ground in these areas, and remains thicker than the other portions. On the other hand, when hard protrusions such as bumps are present, the wafer is ground to a greater degree and thus becomes thinner than the other portions. This phenomenon is not a problem if the finished thickness is 150 μm or greater, but if the wafer is thinned to less than 100 μm (especially when finishing to a thickness of 50 μm or less), or if tall protrusions such as bumps are present on the wafer circuit side (for example, at 100 μm or greater), not only is the transverse strength of the wafer reduced considerably, but in severe cases the wafer can even be damaged during grinding. Also, when a wafer is thinly ground to about 50 μm edge chipping of the wafer or penetration of grinding water between the wafer and surface protecting layer can constitute a problem, but this has also been caused by a lack of adhesion of the surface protecting sheet to the wafer edges. In addition, with semiconductor wafers which are thin and also have bumps or other protrusions of 100 μm or greater on the circuit side, it is difficult to accomplish grinding by attachment of typical surface protecting sheets.
A conventional surface protecting sheet is usually a sheet having a pressure-sensitive adhesive layer as a surface protecting layer on a polymeric film material. The pressure-sensitive adhesive is designed so as to have a low elastic modulus in order to follow irregularities on the circuit side. If this tendency is too strong, however, the wafer can undergo considerable stress and suffer damage when the sheet is removed by peeling from the wafer. This has led to the development of energy beam peelable protecting sheets wherein the pressure-sensitive adhesive is hardened by irradiation with an energy beam such as ultraviolet rays to lower the adhesive strength between the wafer and protecting sheet before peeling. However, an often occurring problem is that the pressure-sensitive adhesive layer is too flexible in the unhardened state during grinding, and the wafer thus suffers damage during grinding.
Japanese Unexamined Patent Publication HEI No. 11-26404 discloses a wafer grinding method whereby the aforementioned energy beam peelable protecting sheet is attached to a circuit-formed wafer and the pressure-sensitive adhesive layer is hardened with an energy beam, after which the back side of the wafer is ground. However, since the pressure-sensitive adhesive is not fluid, its conformability against irregularities on the wafer circuit side is not adequate.
Japanese Unexamined Patent Publication No. 2000-38556 discloses a hot-melt semiconductor surface protecting sheet. The hot-melt sheet melts and exhibits fluid properties by heating at 60-100° C., and thus is able to conform to irregularities on the circuit side and exhibit excellent grinding properties. However, the sheet repeatedly melts when the temperature rises above the melting point. In most cases, a semiconductor wafer which has been laminated with a protective sheet is subjected to subsequent heating during lamination of a film used for attachment of chips, or a “die attachment film” (hereinafter referred to as “DAF”), or during a metal film forming step by sputtering. As a result, the temperature increase produced in such steps has caused problems including remelting of the protective sheet.